Zeolite membrane having afx structure, membrane structure, and method for manufacturing membrane structure

ABSTRACT

A peak intensity of a (110) plane is greater than or equal to 2.5 times a peak intensity of a (004) plane in an X-ray diffraction pattern obtained by irradiation of X-rays to a membrane surface of the AFX membrane.

TECHNICAL FIELD

The present invention relates to a zeolite membrane having an AFXstructure, a membrane structure, and a method for manufacturing amembrane structure.

BACKGROUND ART

In recent years, techniques for separating and concentrating desiredcomponents from a gas mixture or a liquid mixture using zeolitemembranes have been proposed.

Specifically, a zeolite membrane having a DDR structure, a zeolitemembrane having an LTA structure, a zeolite membrane having an FAUstructure, a zeolite membrane having an MFI structure, and a zeolitemembrane having a CHA structure are known as zeolite membranes for gasseparation, for example (see WO 2013/125660).

Furthermore, a zeolite membrane having an LTA structure, a zeolitemembrane having an MOR structure, a zeolite membrane having an FERstructure, and a zeolite membrane having a CHA structure are known aszeolite membranes for liquid separation, for example (see WO2013/125660).

SUMMARY

However, there has been no report indicating success in forming zeolitemembranes having an AFX structure.

Development of practicable zeolite membranes having an AFX structurehave been expected.

The present invention was made in light of the above-describedcircumstances, and aims to provide a practicable zeolite membrane havingan AFX structure, and a method for manufacturing the same.

The peak intensity of a (110) plane is 2.5 times or more the peakintensity of a (004) plane in an X-ray diffraction pattern obtained byirradiation of X-rays to a membrane surface of a zeolite membrane havingan AFX structure according to the present invention.

According to the present invention, it is possible to provide a zeolitemembrane having an AFX structure, a membrane structure, and a method formanufacturing a membrane structure that are practicable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a zeolite membrane having an AFXstructure.

FIG. 2 is a plane view of a zeolite membrane having an AFX structure.

FIG. 3 is a diagram illustrating a method for manufacturing a zeolitemembrane having an AFX structure.

FIG. 4 is a diagram illustrating a method for manufacturing a zeolitemembrane having an AFX structure.

DESCRIPTION OF EMBODIMENTS

Membrane Structure 1

FIG. 1 is a cross-sectional view of a membrane structure 1. FIG. 2 is aplane view of a zeolite membrane 10 having an AFX structure.

The membrane structure 1 includes a porous support 10 and a zeolitemembrane 20 having an AFX structure. The zeolite membrane 20 having anAFX structure is constituted by zeolite crystals 30 having an AFXstructure.

In the following description, the zeolite membrane 20 having an AFXstructure is abbreviated as an “AFX membrane 20”, and the zeolitecrystals 30 having an AFX structure are abbreviated as “AFX crystals30”.

1. Porous Support 10

The porous support 10 supports the AFX membrane 20. The porous support10 has chemical stability to an extent that the AFX membrane 20 can beformed (crystallized, applied, or deposited) on a surface of the poroussupport 10 in the form of a membrane.

The porous support 10 is a ceramic sintered body. Alumina, silica,mullite, zirconia, titania, yttria, silicon nitride, silicon carbide,ceramic sand, cordierite, or the like can be used as the aggregate ofthe porous support 10. The porous support 10 may contain a bindingmaterial. A glass material containing silicon (Si), aluminum (Al),titanium (Ti), or the like can be used as the binding material. Thecontent of the binding material may be set to be 20 vol % or more and 40vol % or less, but is not limited thereto.

The porous support 10 need only have a shape according to which a fluidmixture (gas mixture or liquid mixture) to be subjected to separationcan be supplied to the AFX membrane 20. Examples of the shape of theporous support 10 include a monolith-shape, a flat plate shape, atubular shape, a cylindrical shape, a columnar shape, and a prismaticshape. A monolith-shape refers to a shape having a plurality of cellsprovided in the longitudinal direction, and has a honeycomb shape. Ifthe porous support 10 has a monolith-shape, the length thereof in thelongitudinal direction can be set to 150 to 2000 mm, and the diameterthereof in the radial direction can be set to 30 to 220 mm, but there isno limitation thereon. If the porous support 10 has a monolith-shape, itis possible to form 30 to 2500 channels having a diameter of 1 to 5 mmin the porous support 10.

The porous support 10 is a porous body having multiple open pores. Anaverage pore size of the porous support 10 need only be a size at whicha permeation component of the fluid mixture that has permeated throughthe AFX membrane 20 can pass through pores. The amount of a permeationcomponent can be increased by increasing the average pore size of theporous support 10. The strength of the porous support 10 can beincreased by reducing the average pore size of the porous support 10.The average pore size of the porous support 10 is not particularlylimited, and can be 0.01 μm or more and 5 μm or less, for example. Theaverage pore size of the porous support 10 can be measured depending onthe size of pores using a mercury intrusion method, an air-flow methoddescribed in ASTM F316, or perm porometry. The porosity of the poroussupport 10 is not particularly limited, and can be 25% to 50%, forexample.

The average particle size of the porous support 10 is not particularlylimited, and can be 0.1 μm or more and 100 μm or less, for example. Theaverage particle size of the porous support 10 refers to an arithmeticaverage value of the maximum diameters of 30 particles that are measuredthrough cross-sectional observation using a SEM (Scanning ElectronMicroscope). 30 particles to be measured need only be selected in a SEMimage at random.

The porous support 10 may have a monolayer structure in which pores havea uniform size, or a multilayer structure in which pores have differentsizes. If the porous support 10 has a multilayer structure, it ispreferable that the closer a layer is to the AFX membrane 20, thesmaller the average pore size is. If the porous support 10 has amultilayer structure, the average pore size of the porous support 10refers to an average pore size of an outermost layer that is in contactwith the AFX membrane 20. If the porous support 10 has a multilayerstructure, each layer can be constituted by at least one selected fromthe above-described materials, and materials constituting layers may bedifferent from each other.

2. AFX Membrane 20

The AFX membrane 20 is formed on a surface of the porous support 10. Thethickness of the AFX membrane 20 is not particularly limited, and can beset to 0.1 μm or more and 10 μm or less. The AFX membrane 20 preferablyhas a thickness of 0.3 μm or more, and more preferably has a thicknessof 0.5 μm or more, in consideration of sufficiently bonding crystals.The AFX membrane 20 preferably has a thickness of 5 μm or less, and morepreferably has a thickness of 3 μm or less, in consideration ofsuppressing cracking caused by thermal expansion.

The AFX membrane 20 is formed in the form of a membrane as a result of aplurality of AFX crystals 30 being linked to each other. Each AFXcrystal 30 is a crystal constituted by a zeolite having an AFXstructure. The AFX structure refers to a type of structure that meetsthe definition of an AFX type structure under the IUPAC structure codesas defined by the Structure Commission in the International ZeoliteAssociation.

Examples of zeolites constituting AFX crystals 30 include a zeolite inwhich atoms (T atoms) located at centers of oxygen tetrahedrons (TO₄)constituting the zeolite are constituted by Si and Al, an AlPO zeolitein which T atoms are constituted by Al and P (phosphorus), an SAPOzeolite in which T atoms are constituted by Si, Al, and P, an MAPSOzeolite in which T atoms are constituted by magnesium (Mg), Si, Al, andP, and a ZnAPSO zeolite in which T atoms are constituted by zinc (Zn),Si, Al, and P. A portion of T atoms may be substituted by otherelements.

Each AFX crystal 30 internally has a plurality of oxygen 8-membered ringpores. An oxygen 8-membered ring pore refers to a pore constituted by anoxygen 8-membered ring. An oxygen 8-membered ring is also simplyreferred to as an “8-membered ring”, and is a portion in which thenumber of oxygen atoms constituting the pore framework is eight, andoxygen atoms are linked to the above-described T atoms to form a ringstructure.

Each AFX crystal 30 may contain a metal or metal ion for the purpose ofproviding a specific component with adsorptivity. Examples of such ametal or metal ion include one or more selected from the groupconsisting of alkali metals, alkaline earth metals, and transitionmetals. Although specific examples of transition metals include platinum(Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), copper (Cu),cobalt (Co), manganese (Mn), and indium (In), there is no limitationthereon.

Each AFX crystal 30 is formed in a plate shape. Although there is noparticular limitation on a planar shape of each AFX crystal 30 and theplanar shape thereof may be a polygon other than a triangle, or anindeterminate form, a hexagon is particularly preferable. If each AFXcrystal 30 has a hexagonal shape, the AFX crystal 30 has highercrystallinity than that of an AFX crystal having an indeterminate form,a spherical shape, or an elliptical spherical shape, and it is possibleto obtain a membrane having better durability.

As shown in FIGS. 1 and 2, the plate-shaped AFX crystals 30 are disposedextending upward from the surface of the porous support 10. The AFXcrystals 30 are disposed vertically in the thickness direction of theAFX membrane 20. That is, both main faces of the AFX crystals 30 have apredetermined orientation angle with respect to the surface of theporous support 10. An average orientation angle of both main surfaces ofeach AFX crystal 30 with respect to the surface of the porous support 10need only be larger than 60 degrees, and is preferably 70 degrees ormore, and is more preferably 80 degrees or more.

Here, c planes are both main faces of each AFX crystal 30, and a planesare the side surfaces thereof. As described above, the AFX crystals 30are disposed extending upward from the surface of the porous support 10,and thus the a planes, which are side faces, are exposed at the membranesurface of the AFX membrane 20.

The peak intensity of a (110) plane is 2.5 times or more the peakintensity of a (004) plane in an X-ray diffraction pattern obtained byirradiation of X-rays to the membrane surface of the AFX membrane 20using an X-ray diffraction (XRD) method. This means that the abundanceratio of the AFX crystals 30 that are disposed extending upward from thesurface of the porous support 10 is high. Thus, as a result of makingthe peak intensity of the (110) plane be 2.5 times or more the peakintensity of the (004) plane, the c planes, which are the main faces, ofadjacent AFX crystals 30 can be joined to each other, and thus theconnectivity of adjacent AFX crystals 30 can be increased. Thus, it ispossible to inhibit the formation of gaps between AFX crystals 30 andimprove the separation performance of the AFX membrane 20 to apracticable level.

In an X-ray diffraction pattern, the peak intensity of the (110) planeis preferably 3 times or more the peak intensity of the (004) plane, andis more preferably 4 times or more the peak intensity of the (004)plane. This makes it possible to further improve the separationperformance of the AFX membrane 20.

The peak intensity refers to a value obtained by subtracting abackground value from a measured value. An X-ray diffraction pattern canbe obtained by irradiation of CuKα-rays to the membrane surface of theAFX membrane 20 using an X-ray diffraction apparatus (manufactured byRigaku Corporation, model MiniFlex600). The X-ray output is 600 W (tubevoltage: 40 kV, tube current: 15 mA), scan speed is 0.5 degrees/min, ascan step is 0.02 degrees, and an Ni foil having a thickness of 0.015 mmis used as a CuKβ-ray filter. A peak of the (110) plane is observedaround 2θ=13 degrees, and a peak of the (004) plane is observed around2θ=18 degrees.

Method for Manufacturing Membrane Structure 1

1. Preparation of Porous Support 10

A compact is formed by molding a ceramic material into a desired shapeusing an extrusion molding method, a press molding method, a slip castmolding method, or the like.

Then, if the porous support 10 has a multilayer structure, slurrycontaining a ceramic material is applied onto a surface of the compactusing a filtration method or a flow-down method.

The compact is fired (for example, 900 degrees C. to 1450 degrees C.) tothereby form the porous support 10. The porous support 10 may have anaverage pore size of 0.01 μm or more and 5 μm or less.

2. Preparation of Seed Crystals

DDR crystals are synthesized according to a method disclosed in WO2010/90049.

Then, a starting material solution is prepared by dissolving ordispersing T atom sources such as a silicon source, an aluminum source,a phosphorus source or the like and a structure-directing agent (SDA) inpure water. T atoms preferably include two or more of Si, Al, and P, andmore preferably include at least Al, P, and O because the crystallinityof AFX can be improved. Colloidal silica, fumed silica,tetraethoxysilane, sodium silicate, or the like can be used as a siliconsource, for example. Aluminum isopropoxide, aluminum hydroxide, sodiumaluminate, alumina sol, or the like can be used as an aluminum source,for example. Phosphoric acid, sodium dihydrogen phosphate, ammoniumdihydrogen phosphate, or the like can be used as a phosphorus source,for example. N,N,N′,N′-tetramethyldiaminohexane, 1,4-diazabicyclo[2,2,2]octane-C4-diquat dibromide, 1,3-di(1-adamantyl)imidazolium dibromide, orthe like can be used as a structure-directing agent, for example.

Then, hexagonal plate-shaped AFX crystals are synthesized by adding asmall amount of the synthesized DDR crystals to the starting materialsolution, then introducing the resulting mixture to a pressure vessel,and performing hydrothermal synthesis (180 to 200 degrees C., 10 to 100hours). The length of a straight line connecting opposing corners of ahexagonal plate-shaped AFX crystal in a plane view can be set to 0.1 μmto 5 μm, for example.

Then, AFX seed crystals (seed crystals having an AFX structure) areprepared by adjusting the size of the AFX crystals to an extent thatportions of the AFX crystals are locked to openings of pores formed inthe surface of the porous support 10. If an average particle size of thesynthesized AFX crystals is larger than 0.5 times and is smaller than 5times an average pore size of an applied surface of the porous support10, these AFX crystals can be directly used as AFX seed crystals as aresult of dispersing the AFX crystals. If an average particle size ofthe synthesized AFX crystals is larger than 0.5 times an average poresize of the applied surface of the porous support 10, plate-shaped AFXseed crystals (plate-shaped seed crystals having an AFX structure) maybe produced by introducing the synthesized AFX crystals into pure water,and deflocculating and crushing the AFX crystals with use of a ball millor the like so that the average particle size thereof falls within theabove-described range. In crushing, the size of AFX seed crystals can beadjusted by changing the crushing time. The average particle size of theplate-shaped AFX seed crystals can be set to 100 nm to 400 nm, forexample. Also, at this time, DDR crystals may be directly locked to thesurface of the porous support 10, and plate-shaped AFX crystals may bedirectly grown to be disposed extending upward in membrane formation inthe subsequent process. An average particle size of seed crystals ispreferably 0.5 to 5 times an average pore size of the applied surface ofthe porous support 10, and is more preferably 0.7 to 3 times the averagepore size thereof.

3. Formation of AFX Membrane 20

A seed crystal dispersion solution is prepared by dispersing AFX seedcrystals in water, an alcohol such as ethanol or isopropanol, or a mixedsolution thereof.

Then, as a result of filtering the seed crystal dispersion solution ontothe surface of the porous support 10, AFX seed crystals are attached tothe surface of the porous support 10. At this time, as shown in FIG. 3,portions of AFX seed crystals are locked to openings of pores formed onthe surface of the porous support 10, and the AFX seed crystals aresupported in a state of being disposed extending upward from the surfaceof the porous support 10. In order to hold the AFX seed crystals in astate in which the AFX seed crystals are disposed extending upward fromthe surface of the porous support 10, the speed at which the dispersionsolution is filtered is preferably 10 ml/m²·s or more, and is morepreferably 15 ml/m²·s.

Then, a starting material solution is prepared by dissolving ordispersing T atom sources such as a silicon source, an aluminum source,and a phosphorus source, and a structure-directing agent (SDA) in purewater.

Then, the porous support 10 with AFX seed crystals attached is immersedinto the starting material solution and hydrothermal synthesis isperformed (150° to 190 degrees C., 5 to 60 hours). At this time, AFXseed crystals directly undergo crystal growth in a state of beingdisposed extending upward from the surface of the porous support 10, andthus, as shown in FIG. 4, AFX crystals 30 disposed extending upward fromthe surface of the porous support 10 grow, and are joined to each other,thus forming the AFX membrane 20.

EXAMPLES

Examples of the present invention will be described below. However, thepresent invention is not limited to the examples described below.

Example 1

1. Preparation of Porous Support

A monolith-shaped compact having a plurality of through holes was formedfrom a green body containing an alumina raw material by an extrusionmolding method, and then was fired.

Then, a porous layer including alumina as a main component was formed onsurfaces of through holes of the fired compact, and the resultingcompact was fired again to form a porous support. A surface of theporous support on which a membrane is to be formed had an average poresize of 65 to 110 nm.

2. Preparation of Seed Crystals

DDR crystals were synthesized through hydrothermal synthesis (160degrees C., 16 hours) according to the above-described method disclosedin WO 2010/90049, and the synthesized DDR crystals were sufficientlywashed. The DDR crystals had an average particle size of 190 nm. DDRcrystals having low crystallinity were produced by crushing the obtainedDDR crystals with use of a bead mill for 90 minutes.

A starting material solution having a composition of 2.5 SDA:0.75 SiO₂:1Al₂O₃:1.25 P₂O₅:50 H₂O was prepared by dissolving, in pure water,colloidal silica as a silicon source, aluminum isopropoxide as analuminum source, 85% phosphoric acid as a phosphorus source, andN,N,N′,N′-tetramethyldiaminohexane, as a structure-directing agent.

Then, a small amount of DDR crystals was added to the raw materialsolution and the resulting mixture was introduced into a pressurevessel, and hydrothermal synthesis (195 degrees C., 30 hours) wasperformed.

Then, crystals obtained through hydrothermal synthesis were collectedand sufficiently washed with pure water, and then were completely driedat 65° C.

Then, when a crystal phase was checked through X-ray diffractionmeasurement, and outer shapes of crystals were checked using a SEM, theobtained crystals were hexagonal plate-shaped AFX crystals. A straightline connecting opposing corners of an AFX crystal in a plane view had alength of 1 to 5 μm.

Then, plate-shaped AFX seed crystals were produced by introducing theAFX crystals into pure water and crushing the AFX crystals with use of aball mill for 7 days. A straight line connecting opposing corners of aplate-shaped AFX seed crystal in a plane view had a length ofapproximately 200 nm.

3. Formation of AFX Membrane

A seed crystal dispersion solution was prepared by dispersing the AFXseed crystals in ethanol.

Then, as a result of filtering the seed crystal dispersion solutionthrough channels of the porous support, AFX seed crystals were attachedto inner surfaces of the channels of the porous support. As describedabove, because the porous support had an average pore size of about 100nm, and a plate-shaped AFX seed crystal had a diagonal length of 200 nm,AFX seed crystals were locked to and disposed extending upward fromopenings of pores of the porous support through filtration at 35ml/m²·s.

Then, a starting material solution having a composition of 1.7 SDA:0.75SiO₂:1 Al₂O₃:1.25 P₂O₅:305 H₂O was prepared by dissolving, in purewater, colloidal silica as a silicon source, aluminum isopropoxide as analuminum source, 85% phosphoric acid as a phosphorus source, andN,N,N′,N′-tetramethyldiaminohexane as a structure-directing agent.

An AFX membrane was synthesized by immersing the porous support with AFXseed crystals attached into the starting material solution andperforming hydrothermal synthesis (170 degrees C., 50 hours).

Then, the synthesized AFX membrane was sufficiently washed with purewater, and then was completely dried at 90 degrees C. After drying, theN₂ permeation amount of the AFX membrane was measured and found to be0.6 nmol/m²·s·Pa or less. Accordingly, it was confirmed that the AFXmembrane according to Example 1 had a practicable degree of denseness.

Then, SDA was burned off through heat treatment at 500 degrees C. for 20hours so that pores passed through the AFX membrane.

Then, in a separation test using a mixed gas of CO₂/CH₄ (50:50) at 0.2MPaG, the AFX membrane, of which both end portions of the porous supportwere sealed with a sealing material, demonstrated a CO₂/CH₄ permeanceratio of 159. Similarly, in a separation test using a mixed gas ofN₂/CH₄ (50:50) at 0.3 MPaG, the AFX membrane, of which both end portionsof the porous support were sealed with a sealing material, demonstrateda N₂/CH₄ permeance ratio of 6.3. Accordingly, it was confirmed that theAFX membrane according to Example 1 had sufficiently practicableseparation performance.

The peak intensity of the (110) plane was 4.1 times the peak intensityof the (004) plane in an X-ray diffraction pattern obtained byirradiation of X-rays to the AFX membrane surface. Accordingly, it wasconfirmed that in the AFX membrane according to Example 1, a planes ofthe AFX crystals were arranged on the membrane surface.

Example 2

1. Preparation of Porous Support

A porous support was prepared in the same process as that of Example 1.

2. Preparation of Seed Crystals

AFX seed crystals were prepared in the same process as that of Example1.

3. Formation of AFX Membrane

An AFX membrane was synthesized in the same process as that of Example1, except that the composition of the starting material solution waschanged to 1 Al₂O₃:2.1 P₂O₅:2.8 SDA:850 H₂O, and hydrothermal synthesisconditions were changed to 170 degrees C. for 45 hours.

Then, the synthesized AFX membrane was sufficiently washed with purewater, and then was completely dried at 90 degrees C. After drying, theN₂ permeation amount of the AFX membrane was measured and found to be0.06 nmol/m²·s·Pa or less. Accordingly, it was confirmed that the AFXmembrane according to Example 2 had sufficiently practicable denseness.

Then, SDA was burned off through heat treatment at 450 degrees C. for 50hours so that pores passed through the AFX membrane.

Then, in a separation test using a mixed gas of CO₂/CH₄ (50:50) at 0.15MPaG, the AFX membrane, of which both end portions of the porous supportwere sealed with a sealing material, demonstrated a CO₂/CH₄ permeanceratio of 94. Similarly, in a separation test using a mixed gas of N₂/CH₄(50:50) at 0.3 MPaG, the AFX membrane, of which both end portions of theporous support were sealed with a sealing material, demonstrated aN₂/CH₄ permeance ratio of 3.5. Accordingly, it was confirmed that theAFX membrane according to Example 2 had sufficiently practicableseparation performance.

The peak intensity of the (110) plane was 3.3 times the peak intensityof the (004) plane in an X-ray diffraction pattern obtained byirradiation of X-rays to the AFX membrane surface. Accordingly, it wasconfirmed that in the AFX membrane according to Example 2, a planes ofthe AFX crystals were arranged on the membrane surface.

Example 3

1. Preparation of Porous Support

A porous support was prepared in the same process as that of Example 1.

2. Preparation of Seed Crystals

DDR crystals used to produce the hexagonal plate-shaped AFX seedcrystals in Example 1 were directly filtered, and were applied to thesurface of the porous support.

3. Formation of AFX Membrane

An AFX membrane was synthesized in the same process as that of Example2, except that hydrothermal synthesis conditions were changed to 185degrees C. for 50 hours.

Then, the synthesized AFX membrane was sufficiently washed with purewater, and then was completely dried at 90 degrees C. After drying, theN₂ permeation amount of the AFX membrane was measured and found to be0.08 nmol/m²·s·Pa. Accordingly, it was confirmed that the AFX membraneaccording to Example 3 had sufficiently practicable denseness.

Then, SDA was burned off through heat treatment at 450 degrees C. for 50hours so that pores passed through the AFX membrane.

Then, in a separation test using a mixed gas of CO₂/CH₄ (50:50) at 0.15MPaG i, the AFX membrane, of which both end portions of the poroussupport were sealed with a sealing material, demonstrated a CO₂/CH₄permeance ratio of 133. Similarly, in a separation test using a mixedgas of N₂/CH₄ (50:50) at 0.3 MPaG, the AFX membrane, of which both endportions of the porous support were sealed with a sealing material,demonstrated a N₂/CH₄ permeance ratio of 4.6. Accordingly, it wasconfirmed that the AFX membrane according to Example 3 had sufficientlypracticable separation performance.

The peak intensity of the (110) plane was 3.9 times the peak intensityof the (004) plane in an X-ray diffraction pattern obtained byirradiation of X-rays to the AFX membrane surface. Accordingly, it wasconfirmed that in the AFX membrane according to Example 3, a planes ofthe AFX crystals were arranged on the membrane surface.

1. A zeolite membrane having an AFX structure, wherein a peak intensityof a (110) plane is greater than or equal to 2.5 times a peak intensityof a (004) plane in an X-ray diffraction pattern obtained by irradiationof X-rays to a membrane surface of the zeolite membrane.
 2. The zeolitemembrane having an AFX structure according to claim 1, wherein the peakintensity of the (110) plane is greater than or equal to 3 times thepeak intensity of the (004) plane.
 3. The zeolite membrane having an AFXstructure according to claim 1, the zeolite membrane comprising two ormore of Si, Al, and P.
 4. The zeolite membrane having an AFX structureaccording to claim 3, the zeolite membrane comprising at least Si, Al,P, and O.
 5. The zeolite membrane having an AFX structure according toclaim 3, the zeolite membrane comprising at least Si, Al, and O.
 6. Amembrane structure comprising: the zeolite membrane having an AFXstructure according to claim 1; and a porous support configured tosupport the zeolite membrane having an AFX structure.
 7. A method formanufacturing a membrane structure, comprising: attaching plate-shapedseed crystals having an AFX structure to a surface of a porous support;and immersing the porous support into a starting material solution andperforming hydrothermal synthesis, wherein the plate-shaped seedcrystals having an AFX structure are disposed extending upward from thesurface of the porous support in the step of attaching the plate-shapedseed crystals having an AFX structure to the surface of the poroussupport.